Alcohols have a variety of applications in industry and science. For example, alcohols can be used as a beverage (i.e, ethanol), fuel, reagents, solvents, and antiseptics. For example, butanol is an alcohol that is an important industrial chemical with a variety of applications, including use as a fuel additive, as a feedstock chemical in the plastics industry, and as a food-grade extractant in the food and flavor industry. Accordingly, there is a high demand for alcohols, such as butanol, as well as for efficient production methods which do not rely on non-renewable resources.
Production of alcohol utilizing fermentation by microorganisms is one such production method which utilizes substrates from renewable feedstocks. In the production of butanol in particular, some microorganisms that produce butanol in high yields also have low butanol toxicity thresholds, such that butanol needs to be removed from the fermentation vessel as it is being produced. Thus, there is a continuing need to develop efficient methods and systems for producing butanol in high yields despite low butanol toxicity thresholds of the butanol-producing microorganisms in the fermentation medium. In situ product removal (ISPR) (also referred to as extractive fermentation) can be used to remove butanol (or other fermentative alcohol) from the fermentation vessel as it is produced, thereby allowing the microorganism to produce butanol at high yields. One method for ISPR for removing fermentative alcohol that has been described in the art is liquid-liquid extraction (U.S. Patent Appl. Pub. No. 20090305370). In general, with regard to butanol fermentation, for example, the fermentation medium, which includes the microorganism, is contacted with an organic extractant. The organic extractant and the fermentation medium form a biphasic mixture. The butanol partitions into the organic extractant phase, decreasing the concentration in the aqueous phase containing the microorganism, thereby limiting the exposure of the microorganism to the inhibitory butanol. Liquid-liquid extraction results from contact between the extractant and the fermentation broth for transfer of the product alcohol into the extractant; separation of the extractant phase from the aqueous phase; and, preferably, recycle of the extractant with minimal degradation of the partition coefficient of the extractant over a long-term operation.
The extractant can become contaminated over time with each recycle by, for example, the build-up of lipids present in the biomass that is fed to the fermentation vessel as feedstock of hydrolysable starch. As an example, a liquified corn mash loaded to a fermentation vessel can result in a fermentation broth that contains corn oil during conversion of glucose to butanol by simultaneous saccharification and fermentation (with saccharification of the liquified mash occurring during fermentation by the addition of glucoamylase to produce glucose). The dissolution of the corn oil lipids into an extractant during ISPR can result in build-up of lipid concentration with each extractant recycle, decreasing the partition coefficient for the product alcohol in extractant as the lipid concentration in extractant increases with each recycle.
Converting the lipids present in a liquefied mash into an extractant that can be used in ISPR is a method of decreasing the amount of lipids that are fed to the fermentation vessel, as is esterifying the product alcohol as it is produced during the fermentation with a fatty acid by adding lipase as an esterification catalyst to the fermentation. Such methods are described for example in US Appl. Pub. Nos. 20110312044 and 20110312043, and PCT Appl. Pub. No. WO2011/159998
There is a continuing need for alternative extractive fermentation methods which can also reduce costs associated with adding lipase to the fermentation.